Catalytic effect of hydrogen chloride on the degradation of poly(vinyl chloride)

The distribution of polyenes which results from the chemical dehydrochlorination of poly(vinyl chloride) has been studied in dichloromethane (DCM) and tetrahydrofuran (THF) solvents. A higher percentage of longer polyenes is formed in DCM than in THF. On the addition of trifluoroacetic acid to DCM solutions of the polyenes, new species are formed with strong absorptions in the region 500–800 nm. The absorptions are probably due to polyenylic ions formed by protonation of the polyenes, and the interrelated changes in their intensities can be explained by the migration of short polyene sequences along the polymer chain with the formation of longer sequences. The species are extremely photosensitive and are bleached in a few seconds with light from a medium-pressure mercury lamp. The relevance of these experiments to the effect of HCl on the polyene distribution and on the rate of photocrosslinking is discussed.     

Partially dehydrochlorinated PVC. I. Structure

PVC made by suspension and anionic polymerization was partially dehydrochlorinated by three different methods. In the first one, a solution in THF and in the second one, particles swollen by dioxane were treated with alcoholic KOH at low temperatures. In the third method, a solution of PVC in DMF was heated to 130°C or refluxed (153°C). The reactions were followed by UV spectrophotometry and the products analyzed by GPC. Furthermore, the products were ozonolyzed and the molecular weights of the polymeric residues determined by GPC. Treatment with KOH in THF yielded products with randomly distributed polyene sequences. At higher degrees of dehydrochlorination a slight decrease in molecular weight was observed. The polymers were shown to be built up by PVC segments with an average degree of polymerization between 70 and 80, interspaced by polyene segments each with relatively few conjugated double bonds in sequence (fewer than 15). The reaction of KOH with particles swollen in dioxane was slower but yielded products with the same molecular structure. Thermal dehydrochlorination in DMF gave rise to long polyene sequences. They were fewer in number as evidenced by the higher molecular weights of the residues after ozonolysis.     

On the thermal degradation of poly(vinyl chloride). III. Structural changes during degradation in nitrogen

The structural changes in poly(vinyl chloride) during thermal degradation in nitrogen at 190°C have been investigated. From gel permeation chromatography analyses no chain scission, but only crosslinking reactions were observed. An increase in the molecular weight was measured even at 0.3% conversion. For longer polyene sequences and at higher conversions, a crosslinking reaction competed with the “zipper” propagation. The secondary reactions, were more extensive at longer polyene sequence lengths. The growing polyene sequences can be terminated not only by branching reactions but also at existing pendent chloromethylene groups. A decrease in the amount of short chain branching with conversion also indicated other types of secondary reactions. Such a decrease was also observed during thermomechanical degradation in a Brabender Plastograph. The average polyene sequence length was calculated to be around 10, depending somewhat on the type of analysis used. Although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increases appreciably during the early stages of the degradation. Such an increase is, of course, also consistent with a radical mechanism.     

Partially dehydrochlorinated PVC. II. Reactions with dienophiles and hydroxylating agents

Samples of poly(vinylchloride) containing polyene sequences were made by partial dehydrochlorination by alkali in tetrahydrofuran solution, by alkali in dioxane, and thermally in dimethyl-formamide. The reactions of polyene PVC were followed by UV spectrophotometry. Dienophiles were found to have relative reactivities similar to those found in reactions with low molecular-weight dienes. The reaction with maleic anhydride yielded polymers which after hydrolysis contained carboxyl groups. Hydroxylation was made with osmium tetraoxide and performic acid. With the latter reagent the reaction proceeded to complete disappearance of the UV-absorption peaks from trienes and higher polyenes: Gel permeation chromatography analysis showed that hydroxylation could be made with only minor changes in molecular-weight distribution. The formation of gel upon thermal dehydrochlorination in DMF was shown to be due to physical cross linking probably arising by crystallization of polyene segments. The adhesion of the hydroxylated and carboxylated polymers to glass and stainless-steel surfaces was investigated. Films adhered stronger as the degree of dehydrochlorination of the polyene PVC used to make the derivative increased. Samples with long sequences adhered much stronger than those containing short sequences of corresponding degrees of total substitution.     

The influence of the tacticity on thermal degradation of PVC. III. Verification by ozonization of degraded samples

Two sets of ozonization reactions, for 1 h and 24 h respectively, have been carried out at ?20°C on degraded samples of PVC having different contents of tactic sequences. The evolution of the polyene sequences distribution with the ozonization was followed by UV-Visible spectroscopy. The number of chain scissions of ozonized samples was calculated from the number average molecular weight measurements before and after ozonization. The observed increase of short polyenes relative to the long polyenes with ozonization was found to depend markedly on the content of syndiotactic sequences. On the other hand, the number of chain scissions after total ozonization was found to be the higher the lower the content of tactic sequences in PVC samples is.     

Assignment of UV-absorption maxima of degraded PVC

Summary UV-spectra of degraded PVC are put together in a complicated but surveyable way by superposition and deletion of the a-, b-, c-, and d-absorptions of individual polyenes. The existing conception according to which all maxima are based on the most intensive longest-wave d-absorption of a definite polyene sequence could not be confirmed. It has been established that all spectra are showing clearly two additional maxima resulting from a superposition of other polyene absorptions. Thus, counting all maxima for d-absorptions, in the literature a wrong assignment of the UV-maxima to the individual polyenes is reported.Furtheron, it is shown that the frequency distribution of the polyene sequence length is not influenced by the extent of PVC-degradation, but is mainly depending on the degradation temperature.     

Influence of tacticity on thermal degradation of PVC. V. Relation between the nature of labile conformations and the polyene distribution in the degraded polymer

Four poly(vinyl chloride) (PVC) samples, 1, 2, 3, and 4, were prepared in bulk using 2,2′-azodiisobutyronitrile at 90°C and 60°C and the same initiator together with UV irradiation at 0°C and ?50°C, respectively. Fractions were obtained from samples 2,3, and 4 by extracting with acetone, sample 1 being completely soluble in this solvent. The whole PVC's as well as their acetone soluble and insoluble fractions, were characterized by determining the intrinsic viscosity, the osmometric molecular weight, and the tacticity; then they were thermally degraded up to conversion of 0.3% in powder state. The values of degradation rate for both the insoluble fractions and the whole PVC's were in agreement with some prior results on the influence of syndiotactic sequences on propagation step. The soluble fractions proved to be very unstable in comparison with the insoluble, which, despite the lower molecular weight of the former, seems to obey their higher content of isotactic conformations. The fine polyene distribution in the degraded polymers was carried out by UV–visible spectroscopy. The results reveal the occurrence of two different polyene distributions, and allow for the peculiar one of the soluble fractions to be related to polyenes consisting of two sequences of trans conjugated double bonds separated by a single cis double bond. The results clearly shows that there are two mechanisms for initiation of the PVC degradation, depending on whether it occurs by random unstable structures or by the normal GTTG isotactic or TTTG heterotactic triads. Moreover, these two initiation processes are proved to give rise to different types of polyenes, which accounts for the occurrence of two unlike mechanisms of propagation.     

Stabilization of poly(vinyl chloride). IV. Color changes on heating poly(vinyl chloride)–dye systems

The stabilization of poly(vinyl chloride) (PVC) involving complementary colors has been previously reported. Obliterating polyene color with various dyes containing complementary colors with the polyene color is studied on the basis of colorimetry. The changes in the color of heated PVC containing Thren Blue IRN, Ceres Blue GN, Oplas Violet 730, Macro-Lex Violet 3R, Macro-Lex Green 5B, or Macro-Lex Red 5B were investigated using a differential colorimeter. When the PVCs containing various dyes were heated, the discoloration from the color of each dye to the color mixture of each dye and polyene color was observed with increased heating times for all systems. In particular, an achromatic color has been observed, during the heat treatments, in PVC containing blue dyes such as Thren Blue IRN or Ceres Blue GN, which set up complementary color relationship with the polyene color. Thus the color of polyenes, which appears with advancing dehydrochlorination of PVC, is masked by the blue dyes. It is also apparent that the obliteration of polyene color does not depend on the chemical influences of the dyes added, but by color mixing of polyenes and the dyes. Cool-color dyes markedly slow down the appearance of polyene colors.     

Structural changes of chlorinated paraffins at processing conditions of particulate filled polypropylene

Structural changes of chlorinated paraffin (CP) have been studied using UV-visible absorption spectroscopy in the 180 to 220°C temperature range. Formation and further reaction of conjugated polyene sequence with and without MgO is of major concern. It was found that CP dehydrochlorinated at normal processing temperatures of polypropylene in the absence of MgO. A mixture of various polyene sequences was formed. Unlike a stabilizing effect on poly(vinyl chloride), MgO showed a catalytic effect for dehydrochlorination of CP. As a result, longer conjugation was formed with MgO at the same conditions except for the absence of MgO. The longer polyenes were selectively reacted at the conditions studied. A possible chemical bond formation at the particulate filler/polypropylene matrix interface was suggested as one of the reinforcement mechanisms of the composite.     

Characterization of acetylene terminated sulfone (ATS) resin. II. Thermal analysis of the resin

The kinetics and thermodynamics of the cure and post-cure reactions of an acetylene terminated sulfone (ATS) resin mixture and pure fractions from that mixture were studied using differential scanning calorimetry. Thermal stabilities were measured by programmed thermogravimetry under helium, and oxidative stabilities using isothermal thermogravimetry in air. Microstructural changes accompanying the cure reaction were identified using infrared spectroscopy. Early products of thermal degradation were trapped for identification and quantitative analysis using the technique of sub-ambient thermal volatilization analysis. We found that glass transition temperatures increase with crosslink density in the resin, that oxygen is able to copolymerize or couple with the polyenes to produce peroxides or hydroperoxides (respectively) which subsequently decompose to form terminal alcohols on the polyene, that weight loss in air at 600°F is an oxidative process that is insensitive to resin crosslink density, and that sulfone functionality constitutes the thermal weak link in the ATS resin system.     

Effect of metal stearate stabilizers on the thermal degradation of PVC in solution: The reversible blocking mechanism of stabilization

Systematic thermal degradation studies of PVC solutions have been carried out in the presence of different metal carboxylates (Pb-, Cd-, Ba-, Ca- and Zn- stearates). ZnSt₂ markedly accelerates degradation of the polymer. Significant induction periods (t_i) for the appearance of free HCI are obtained in the presence of the other salts. CaSt₂ acts mainly as an HCI-scavenger and has no direct influence on the elimination process. Mainly short polyenes are formed during the induction period when PbSt₂, CdSt₂, and BaSt₂ are used. After consumption of stabilizers rapid HCI loss occurs, and suddenly longer polyenes are formed, i.e., the amount of double bonds and the average length of polyenes sharply increases. At the same time, the rate of initiation (i.e., the rate of the polyene sequence formation) does not change at all around t_i. None of the investigated stabilizers retard effectively the random initiation of HCI loss. In the presence of PbSt₂, BaSt₂, and CaSt₂, and CaSt₂, the initiation rate is nearly identical to that of the unstabilized PVC. The concentration of labile sites (h₀) in PVC at the end of the induction period has been estimated by kinetic analysis. It has been found that in some cases h₀ is higher than the concentration of labile structures in the virgin PVC. These results indicate that the main role of metal soap stabilizers is the blocking of the fast zip-elimination of HCI from the PVC chain. It is likely that blocking occurs by attachment of a carboxylate group at the end of a propagating zip. This is a reversible process: the blocked structures become active again mainly after the consumption of stabilizers presumably by HCI-catalyzed hydrolysis. In contrast to other stabilization mechanisms by Frye and Horst, Minsker and coworkers, and Michell the reversible blocking mechanism of PVC stabilization is able to explain the experimental findings presented in this study. It also explains such facts of practical importance as color stability of the resin during the induction period and fast blackening after the consumption of stabilizers.     

Thermal Degradation of Vinylchloride/α-Olefin Copolymers

Abstract

The preliminary results obtained for the thermal degradation of vinylchloride/α-olefin copolymers are, in part, unexpected. Decrease of the initial polyene length with the increase of comonomer content is a reasonable consequence of the shorter viny chloride sequence lengths, but the constancy of these lower values in the course of further degradation indicates, that some secondary reactions of the polyenes (e.g. cyclization) must be absent here. The decrease of the HCl elimination rate in case of copolymers differs from the dependence given by the weak sites model. Formally, the rate is proportional to a netative power of conversion, the value of the exponent depending on the composition. These results indicate the necessity of further research for clearing up the unknown details of PVC degradation mechanism.     

On the thermal degradation of poly(vinyl chloride). II. The effect of atmosphere

The early stages of the thermal degradation of PVC were studied. Two commercial, suspension-polymerized resins were thermally treated at different temperatures and oxygen contents. Dehydrochlorination kinetics were followed by conductometric measurements and the formation of polyene sequences by ultraviolet-visible spectroscopy. Crosslinking and chain scission were followed by gel chromatography (GPC) and viscometry. No chain scission was observed in nitrogen atmosphere and no crosslinking in oxygen. Degradation in air proceeded by both reactions. The rate of dehydrochlorination for one of the investigated polymers increased linearly with the logarithm of the oxygen pressure. In nitrogen, an increasing degradation temperature was found to give both an increasing crosslinking and less discoloration. In oxygen, chain scission reactions showed a slight temperature dependence. The temperature effect on the discolorations was similar to that in nitrogen. The main difference between the investigated resins was the amount of internal doubled bonds in the original polymers, the ratio being 2:1. The higher amount resulted in a higher rate of dehydrochlorination, a larger extent of chain scission in oxygen, and a lower extent of crosslinking in nitrogen. Both in oxygen and nitrogen, the obtained results are consistent with allylic mechanisms. In nitrogen, the polyene formation supposedly proceeds by a unimolecular reaction and crosslinking by an intermolecular nonradical dehydrochlorination. In oxygen, radical reactions are superposed and may lead to chain rupture via β-scissions of alkoxy radicals.     

Demonstrating a New Valence-Bond Interpretation of Conjugation in Polycyclic Conjugated Hydrocarbons with a Single Kekulé Structure

For polyene molecules with a single classical structure, those with more double bonds and more rings have more Dewar resonance structures. The more Dewar resonance structures a polyene has the more it is conjugated and stable. These generalizations are illustrated by comparing sets of molecules with the same number of fixed double bonds. A step-by-step algorithm for rapidly determining the number of Dewar resonance structures in small typical conjugated polyenes is given. Some new analytical equations for the number of Dewar resonance structures (DS) for series of pyrene-related conjugated polyenes with a single classical structure are also presented. Some general qualitative rules for evaluating relative order of conjugation in polyenes are summarized.     

Stabilization of gamma-irradiated poly(vinyl chloride) by epoxy compounds. III. Conjugated double bonds and degree of unsaturation in gamma-irradiated PVC–stabilizer mixtures

The concentration of conjugated polyene sequences was studied in γ-irradiated PVC with 4% admixture of four epoxy stabilizers: diglycidyl ether of 2,2-bis(4-hydroxy-3-methylphenyl)propane (I), styrene oxide (1,2-epoxy ethyl benzene) (IV), epoxidized ricinus oil (VI), and epoxidized soybean oil (Drapex 6.8) (VII). As in the former investigations (Papers I and II), the process of the formation of the polyenes occurs in two stages. The concentration of polyene sequences with n double bonds, H_n the total amount of polyene sequences, ∑H_n, the average length of the polyene sequence, n , and the two extents of reaction x and p, were computed. The stabilizing effect of all compounds used agrees with the increasing content of epoxy groups. The addition of stabilizers diminishes the value of n . The decrease of the fraction of long sequences and the increase of short ones occurs. Apart from the binding of evolved HCl, the protective effect towards the macromolecules of PVC consists mainly in the inhibition of growth of chain dehydrochlorination by the epoxy groups.     

Polyene formation in poly(vinyl chloride) stabilised with pre-heated metal stearates

Summary PVC stabilised with mixtures of pre-heated metal stearates are degraded thermally for specific periods of time, in order to determine their influence on the production of polyenes by uv spectroscopy. Additionally, the PVC samples are treated with NaBH₄ to reduce the carbonylic groups produced during the thermo-oxidative process. Important differences in the polyene formation trends are observed, between the reduced and non-reduced samples, indicating that the carbonyl groups attached to polyenes shift the absorbance maxima and alter considerably the quantification of the double bond conjugations. Received: 14 September 1998/Revised version: 28 January 1999/Accepted: 28 January 1999     

On the thermal degradation of poly(vinyl chloride). IV. Initiation sites of dehydrochlorination

A set of poly(vinyl chloride) samples were investigated with respect to their thermal stability. The dehydrochlorination rates were measured in nitrogen at 190°C by use of a very accurate, conductometric method. For all polymers studied, a significant molecular weight increase was observed after 0.4% conversion. With only one exception, samples exhibiting higher degradation rates showed higher absorptions (350–450 nm) after 0.4% dehydrochlorination. In addition, the relative amount of short polyene sequences was found to be higher for polymers with higher degrees of branching. The dehydrochlorination reaction was predominantly initiated at sites of internal unsaturation (allylic chlorines), but an initiation at tertiary chlorines and unstable end-groups could not be completely omitted. Head-to-head units, extraneous impurities, and syndiotactic sequences were found to be of minor importance in this respect.     

Radiation-induced polyene formation of poly(vinyl chloride). I. Effect of conformations

PVC samples with different syndiotacticities were degraded at temperatures in the range of 10° to 115°C by UV radiation. The average length of polyenes increases with increase in syndiotacticity of PVC. It was found that by comparison with the long tactic sequences, on the basis of Bernoullian statistics, the propagation reaction of the polyenes at room temperature is terminated when there are more than three racemic dyads in the tactic sequences. At room temperature, the deformation of about 1.3 Å of the PVC main chain takes place, and at temperatures above the glass transition temperature the propagation rate of the polyenes is competitive with the relaxation time between the (tt) and the (tg) conformations.     

Thermal degradation of poly(vinyl chloride) in presence of additives,part II. Changes in absorption spectra

Electronic absorption spectra of PVC degraded in ethyl benzoate at 200°C in presence of maleic anhydride (MA), benzoquinone (BQ) und anthraquinone (AQ), were studied. The distribution of polyene sequence length was calculated. It was observed that the rate of formation of polyenes having 6 conjugated double bonds was reduced in presence of MA. In case of BQ and AQ, no such inhibiting effect was observed. However, formation of polyenes having a higher number of double bonds (i. e. n=8) was reduced in the presence of these additives. The ageing behaviour of the degraded polymer samples was also investigated.     

Influence of the experimental conditions on the photooxidation of poly(vinyl chloride)

The photooxidation of poly(vinyl chloride) involves complex reactions in which photooxidation and photodiscoloration by the formation of polyenes compete. These reactions are now fairly well understood, and on the basis of the mechanisms formerly reported, it appears possible to investigate the influence of irradiation parameters such as the thickness of the samples, the temperature, or the external pressure of oxygen. This objective requires the distribution of the photoproducts through the irradiated polymer matrix to be analyzed at the microscopic level. The shapes of the profiles that are determined show that the overall behavior of the polymer results from oxygen-starvation effects. The diffusion of oxygen controls the rate of the photooxidation of the matrix, the rate of the photooxidative bleaching of the polyenes, and the rate of absorption of the light by the residual polyenes that have not been oxidized. © 1993 John Wiley & Sons, Inc.